The standard U.S. Army rifle is the well known M-16 Colt design rifle. Such rifle is fully described and shown in prior U.S. patents.
The rifle can be operated selectively as a semi-automatic rifle. In either operation, the propellant gases formed in the rifle are used to operate the bolt mechanism whereby the bolt is unlocked and then retracted. During retraction, the spent shell is ejected, and a helically wound spring is compressed. The bolt mechanism is then returned to its forward locked position by the spring.
The bolt is operated by the propellant gasses by tapping these gases in the rifle barrel prior to the bullet's leaving the barrel and by transmitting this gas pressure through an elongated tube extending along the top of the barrel to a position adjacent the bolt mechanism. This gas pressure is then used to move the bolt rearwardly slightly during which action the locking mechanism of the bolt is rotated, permitting the bolt to be forced fully rearwardly by direct action from the gases in the barrel, at which time the spent shell is ejected from the chamber of the rifle. Such action is well known.